Smoke sensors have been widely used as fire sensors that detect smoke generated during fire or the like and produce fire alarm. A smoke sensor of a light scattering type is described in Japanese Patent Application Laid Open No. 2001-34862 and a smoke sensor of a light dimming type is described in Japanese Patent Application No. 61-33595.
The smoke sensor of a light scattering type is so configured that scattered light produced when light irradiated by a projection element composed of a light-emitting diode element into a monitoring space is scattered on smoke particles is received by a light receiving element composed of a photodiode. Where smoke particles are present in the monitoring space, the produced scattered light increases the amount of light received by the light receiving element. Therefore, the presence of smoke particles can be detected based on the increase in the amount of light received by the light receiving element. However, because a labyrinth body has to be provided as a measure against stray light, when the flow of air is small, a long time is required for the smoke particles to penetrate into the monitoring space when fire occurs and problems are associated with responsiveness of the sensor.
The smoke sensor of a light dimming type is configured so that light irradiated from a projection element is directly received by a light receiving element, and where smoke particles are present in a monitoring space between the projection element and light receiving element, the amount of light received by the light receiving element decreases and the presence of smoke particle can be detected based on the decrease in the amount of light received by the light receiving element. However, in such sensor, a false alarm (false detection) can be generated under the effect of background light even when no fire has occurred. Furthermore, when a smoke sensor of a light dimming type has a detachable configuration, optical axes of the projection element and light receiving element have to be aligned with high accuracy and problems are associated with time and efforts required for installation.
Furthermore, in the smoke sensors of light scattering type and light dimming type, an alarm can be falsely generated when steam, rather than smoke, penetrates into the monitoring space. Therefore such sensors are not suitable for use in kitchens and bathrooms. Thus, there is also space for improvement in this area.